Sorry wd8ivl, I misread the query.
You cannot change the xy gridlines to dash, only change the colour.
Have a look at this post for some ideas.
http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RPAGEID=135&HOID=50650000000800000097A90000&UCATEGORY_0=_49_%24_6_&UCATEGORY_S=0&USEARCHCONTEXT_TIER_0=0&USEARCHCONTEXT_TIER_S=0&USEARCHCONTEXT_QUESTION_0=grid+dashed&USEARCHCONTEXT_QUESTION_S=0